This invention relates to light-emitting diodes.
Light-emitting diodes are known in which a P-type region is formed in the upper surface of an N-type semiconductor base material. When a voltage is applied across the junction formed by the P-type and N-type materials, photons of radiation are emitted from the boundary, migrate to the surface and escape, the wavelength of the radiation depending on the nature of the P-type and N-type materials employed. For example, if zinc dopant is diffused into Ga As N-type base material to form a P-type region, radiation having a wavelength of about 9,000 A is emitted. Similarly, if an epitaxial layer of Ga-As P is formed on top of a base layer of Ga As and zinc is used as the dopant material, radiation having a wavelength of about 6,000 A is emitted. Since diodes of the latter type emit radiation having a wavelength lying in the visible (red) range of the spectrum, such diodes may be employed as indicating devices or the like.
A critical factor affecting the utility of such light-emitting diodes is the fraction of the emitted radiation which escapes from the material. Since the photons are emitted at the boundary lying predominantly within the surface of the material, they must travel through this material before reaching the viewing surface. In transit, many photons are absorbed in the P layer. These absorbed photons thus make no contribution to the total amount of viewable radiation.
Since the degree of self-absorption of the emitted photons by the material is largely dependent upon the thickness of the P layer, attempts have been made in the past to improve the intensity of the emitted light by reducing the thickness of the P layer to minimize absorption. Such efforts have not been particularly successful due to the fact that the non-radiative recombination of electrons within the P layer increases as the thickness of the P layer decreases. Thus, the emittive capability of the diode decreases as the thickness of the P layer decreases. In practice, it has been found that the minimum thickness of the P region required for successful operation of an LED is on the order of 3 microns. However, P regions with a minimum thickness of 3 microns still exhibit substantial self-absorption of the photons emitted at the boundary and the intensity of the viewable radiation suffers accordingly.
Another factor contributing to the limited intensity characteristics of light-emitting diodes is the radiation pattern exhibited by known light-emitting diodes. It has been found that this radiation pattern approximates a lambertian distribution in which a substantial portion of the radiation emitted from the surface emerges at an angle which is rather shallow with respect to the horizontal surface of the diode. Since the diode is ordinarily viewed at angles close to the normal to the surface, the lambertian distribution correspondingly reduces the component of the emitted radiation normal to the surface. This effect is ordinarily observed as a blurring of the outer edges of a specific character when a plurality of diodes are formed into an alphanumeric array. This undesirable effect has been partially overcome by the use of epoxy lenses which serve to focus a greater portion of the emitted radiation in the normal and near normal directions. Due to the relatively small dimensions of typical P regions, however, and the relatively large size of such lenses, a large proportion of the total emitted radiation is nevertheless lost for viewing purposes. Efforts to overcome the above-noted problems have not met with wide success.